The present invention relates to a rotation angle detector which can be preferably used for the ignition device of an internal combustion engine or the like.
FIGS. 1 and 2 show a major part of conventional rotation angle detector for the distributor of an internal combustion engine. The detector has a shaft 2 which is supported in the central portion of a case 1 so that the shaft is rotated synchronously with the rotation of the engine. A disk 3, which has a prescribed number of through holes 31 at intervals in the circumferential direction of the disk, is secured to the shaft 2 so that the disk is rotated together with the shaft. A light emission element 4 is provided in the face of one side of the disk 3 so that the position of the element corresponds to that of each of the through holes 31 of the disk. A light reception element 5 such as a photodiode is provided in the face of the other side of the disk 3 so that the element receives light emitted from the light emission element 4 and transmitted through the holes 31 of the disk and sends out a signal corresponding to the rotation of the shaft 2. The output terminals of the light reception element 5 are connected to a processing circuit 6 made of an integrated circuit or the like, through a connector not shown in the drawings. The light emission element 4, the light reception element 5 and the processing circuit 6 are integrally assembled together so that they constitute a detection unit 7 secured to the case 1 by a plurality of screws 8, 9, 10.
When the shaft 2 and the disk are rotated together with the engine, the light emitted from the light emission element 4 is received by the light reception element 5 through the holes 31 of the disk as the light is intermittently blocked by the non-holed portion of the disk, so that the light reception element sends out the signal corresponding to the rotation of the shaft. The signal is shaped by the processing circuit 6 so that the rotation angle of the shaft 2 is detected.
However, the conventional rotation angle detector has a problem that the light emission element 4 and the light reception element 5 are likely to be improperly positioned relative to the through holes 31 of the disk 3 on account of three possible causes (i), (ii), (iii). The cause (i) is the positional discrepancy between the shaft 2 and the disk 3. The cause (ii) is the positional discrepancy between the shaft 2 and the case 1. The cause (iii) is the positional discrepancy between the case 1 and the detection unit 7. Since the shaft 2 and the disk 3 are accurately fitted to each other and the case 1 and the shaft 2 are also accurately fitted to each other, the causes (i) and (ii) are substantially eliminated. However, since the detection unit 7 is attached to the case 1 by the plural screws 8, 9, 10 extending through screw holes which are provided in the detection unit so as to define a clearance between the unit and each of the screws to cope with the inaccuracy of the positions of the screw holes, the cause (iii) cannot be eliminated. The positional discrepancy between the case 1 and the detection unit 7 consists of components in the radial and circumferential directions of the case. Since each of the components acts to deteriorate the performance of the rotation angle detector, both the components are undesirable.